1. Technical Field
This application relates to electrosurgical surgery and, in particular, to control systems for electrosurgical generators and to methods of determining tissue moisture content during use of electrosurgical instruments.
2. Description of Related Art
Electrosurgical generators are employed by surgeons in conjunction with an electrosurgical instrument to cut, coagulate, desiccate and/or seal patient tissue. High frequency electrical energy, e.g., radio frequency (RF) energy, is produced by the electrosurgical generator and applied to the tissue by the electrosurgical tool. Both monopolar and bipolar configurations are commonly used during electrosurgical procedures.
Electrosurgical techniques and instruments can be used to coagulate small diameter blood vessels or to seal large diameter vessels or tissue, e.g., soft tissue structures, such as lung, brain and intestine. A surgeon can either cauterize, coagulate/desiccate and/or simply reduce or slow bleeding, by controlling the intensity, frequency and duration of the electrosurgical energy applied between the electrodes and through the tissue. For the purposes herein, the term “cauterization” is defined as the use of heat to destroy tissue (also called “diathermy” or “electrodiathermy”). The term “coagulation” is defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried. “Vessel sealing” is defined as the process of liquefying the collagen and elastin in the tissue so that it reforms into a fused mass with significantly-reduced demarcation between the opposing tissue structures (opposing walls of the lumen). Coagulation of small vessels is usually sufficient to permanently close them. Larger vessels or tissue need to be sealed to assure permanent closure.
In order to achieve one of the above desired surgical effects without causing unwanted charring of tissue at the surgical site or causing collateral damage to adjacent tissue, e.g., thermal spread, it is necessary to control the output from the electrosurgical generator, e.g., power, waveform, voltage, current, pulse rate, etc.
It is known that measuring the electrical impedance and change thereof across the tissue at the surgical site provides a good indication of the state of desiccation or drying of the tissue, e.g., as the tissue dries or loses moisture, the impedance across the tissue rises. This observation has been utilized in some electrosurgical generators to regulate the electrosurgical power based on a measurement of tissue impedance. For example, commonly owned U.S. Pat. No. 6,210,403 relates to a system and method for automatically measuring the tissue impedance and altering the output of the electrosurgical generator based on the measured impedance across the tissue. The entire contents of this patent is hereby incorporated by reference herein.
It has been determined that the particular waveform of electrosurgical energy can be tailored to enhance a desired surgical effect, e.g., cutting, coagulation, sealing, blend, etc. For example, the “cutting” mode typically entails generating an uninterrupted sinusoidal waveform in the frequency range of 100 kHz to 4 MHz with a crest factor in the range of 1.4 to 2.0. The “blend” mode typically entails generating an uninterrupted cut waveform with a duty cycle in the range of 25% to 75% and a crest factor in the range of 2.0 to 5.0. The “coagulate” mode typically entails generating an uninterrupted waveform with a duty cycle of approximately 10% or less and a crest factor in the range of 5.0 to 12.0. In order to effectively and consistently seal vessels or tissue, a pulse-like waveform may be employed. Energy may be supplied in a continuous fashion to seal vessels in tissue if the energy input/output is responsive to tissue hydration/volume through feedback control. Delivery of the electrosurgical energy in pulses allows the tissue to cool down and also allows some moisture to return to the tissue between pulses which are both known to enhance the sealing process.
It is further known to clamp or clip excess voltage output from the electrosurgical generator by the use of avalanche devices, such as diodes, zener diodes and transorbs, resulting in absorption and dissipation of excess energy in the form of heat.
Commonly owned U.S. Pat. No. 6,398,779 discloses a sensor which measures the initial tissue impedance with a calibrating pulse which, in turn, sets various electrical parameters based on a look-up table stored in a computer database. The transient pulse width associated with each pulse measured during activation is used to set the duty cycle and amplitude of the next pulse. Generation of electrosurgical power is automatically terminated based on a predetermined value of the tissue impedance across the tissue.
Commonly owned U.S. Patent Application Publication US 2007/0038209 A1 by Buysse et al., “METHOD AND SYSTEM FOR CONTROLLING OUTPUT OF RF MEDICAL GENERATOR”, the entire contents of which is incorporated by reference herein, discloses a sensor module for sensing (measuring) tissue moisture (which is often indicative of tissue type) and generating a moisture content value and/or determining tissue type. Moisture content is determined from tissue compliance data, e.g., measurement of tissue displacement divided by an applied force, or optical clarity. The additional sensor module may include an infrared or optical sensor for sensing (measuring) light or energy generated by a source, such as an infrared or other light source, which is transmitted through or reflected from the tissue, where the sensed value is indicative of tissue moisture content and/or tissue type of tissue proximate a surgical site. An initial tissue moisture content value and/or tissue type may be provided to a control module as a pre-surgical parameter. Sensed real time moisture content values and/or changes in moisture content over time (Δ(difference) moisture content/Δ(difference) time) may further be provided to the control module during the surgical procedure, where the control module modulates the electrical surgical output in accordance with the sensed real time moisture content values and/or changes in moisture content values over time.
As can be appreciated therefore, since electrosurgical devices are energy based by nature, electrosurgical devices, including radiofrequency (RF), ablation, and similar energy based medical devices have sensors to monitor energy delivery. Such sensors include voltage and current measuring devices. Voltage and current measurement are direct indicators of safety parameters of the electrosurgical instrument and are indirect indicators of tissue impedance and correspondingly of tissue hydration. Tissue hydration affects the degree of cooling and heat transfer available through the local water content of the tissue to prevent overdessication.